A traditional direct current motor essentially comprises two major components: a rotor and a stator, one of which is made of permanent magnet and the other is an electric magnet, and the one is disposed circumferentially by the other. Between a rotor and a stator, there exists an air gap. In one case, an inner rotor rotates within a stator; in another case, an outer rotor rotates around an inner stator. A permanent magnet incorporated on either a rotor or a stator directs a magnetic field into the air gap, which interacts with another magnetic field of changing polarity to develop the torque for driving a motor.
FIG. 1 shows a magnetizing structure that is commonly found in a motor. Such structure of an outer-rotor type motor includes a rotor 12 having a magnet cylinder 121 with smooth surfaces on both sides and a stator 11 having a plurality of silicon steel sheets 111 wound by a plurality of winding coils 13. When a current is applied to a winding coil, an electric magnetic field is created to repulse the magnetic field caused from the permanent magnet, thereby the rotor rotates and drives an article such as a fan.
The permanent magnet is usually in a shape of cylinder; therefore, the term “magnet cylinder” hereinafter means a cylindrical-shaped magnet, unless otherwise specified.
Since the inductive magnetic field in the silicon steel sheets 111 on the stator 11 must reach to an adequate level for starting a rotor, the traditional method includes steps of increasing quantities of winding coils and broadening the size of the silicon steel sheets. Since the current flowing in a motor is considerably higher than the rated load current during the period of starting a rotor, a motor can be easily damaged through overheating. Thus, increasing quantities of winding coils is neither practical nor effective. In addition, the silicon steel sheet 111 on the stator 11 is customarily designed to be asymmetric for changing the air gap between the stator 11 and rotor 12 during the period of starting, as can be seen in FIG. 1. Because such type of silicon steel sheet is costly, broadening its size is generally not suitable. A locked rotor condition is common in the customarily used motor, which also requires special attention.
Therefore, the present invention provides an improved magnetizing structure for overcoming the problems described above.